1. Field of the Invention
The present invention relates to the design of equipment intended for investigating material properties, namely to the apparatus for differential thermal analysis. The invention may prove most advantageous under laboratory conditions for investigating phase transformations in refractory materials.
Though the principle of differential thermal analysis has been known for a comparatively long time and has found wide application in investigating various materials, the problem of increasing accuracy and reliability of results is urgent up to now. In addition, maximum operating temperature of the majority of manufactured devices intended for thermal analysis does not exceed 1800.degree. C. This feature considerably limits measuring capability of such devices due to the absence of the possibility of investigating phase transformations in numerous refractory materials.
2. Description of the Prior Art
Known in the art is an apparatus for differential thermal analysis allowing materials to be investigated at temperatures of up to 2300.degree. C. (see U.S. Pat. No. 3,524,340; British Patent Specification No. 1,133,396; German Pat. No. 1,598,651; French Pat. No. 1,540,445). This apparatus comprises a heated block constructed from a refractory material and defining two chambers. In one of the block chambers there is mounted a crucible for a sample of the material being investigated and in another chamber there is mounted a crucible for a standard material. Hot junctions of a differential thermocouple are brought into contact with bottom portions of the crucibles. Recording instruments such as millivoltmeters and recorders are connected to said thermocouple.
The apparatus above described makes it possible to carry out differential thermal analysis at temperatures of up to 2300.degree. C. Such a temperature limit is dependent on the properties of the thermocouple material. When heating the sample in an electric furnace, some babbles occur in the thermocouple, thus complicating the recording of the parameters being measured.
There is also known in the art an apparatus for differential thermal analysis at higher temperatures (see H. D. Heetderks, E. Rudy, T. Eckert, Planseeberichte fur Pulvermetallurgie, 1965,13,2,105). This apparatus comprises a block constructed from a refractory material and defining two chambers having openings. The openings are provided in covers of the block chambers.
Bottom portions of the chambers are made one-piece and have no openings. In one chamber there is installed a crucible for a sample. In another chamber there is installed a crucible for a standard material. A photoelectric pyrometer is pointed through one opening directly at the sample. Through another opening there is also pointed a photoelectric pyrometer at the standard material. Recording instruments are electrically connected to the outputs of the photoelectric pyrometers.
An obvious advantage of such apparatus consists in the removal of a sensing element from the high temperature zone so as to allow the range of the working temperatures to be significantly expanded. To eliminate the dependence of signals of the photoelectric pyrometers upon the degree of blackness of the object being investigated, blind openings simulating an absolutely black body are provided in the sample and in the standard.
Each photoelectric pyrometer is pointed at the bottom of the corresponding opening in the sample and in the standard. The necessity for simulating an absolutely black body causes some difficulties when using the above described apparatus. It is known that a cylindrical blind opening corresponds to a model of an absolutely black body only when the ratio between the depth and the diameter of said opening is not less than 7. But with such relative dimensions of the openings, the sizes of the sample and of the standard must be sufficiently large while the weight thereof should not be less than 15 g. When investigating alloys based on rare and precious metals, this consideration exerts a considerable influence upon the experimental cost. When investigating hard and brittle materials, the provision of openings in the sample and in the standard entails great processing difficulties. Products of the material evaporation, transferred by convective flows of an inert gas being the medium for heating the block, contribute a significant error to the analysis results. Measuring capability of the apparatus is constricted by the fact that the model of the black body gets damaged during the process of melting the sample. This fact eliminates the possibility of determining the end melting temperature of the sample material and recording temperatures of some phase transformations in the melt, e.g. during the solidification thereof.